1. Field of the Invention
The present invention relates to a shutter apparatus wherein the size or area of a secondary aperture for exposure control changes with the size or area of a primary aperture for exposure photographing, so that the operation of a shutter mechanism is controlled based on an amount of light received through the secondary aperture, and also relates to a camera apparatus adapted to automatically fire a strobe in association with the operation of the shutter apparatus.
2. Description of the Prior art
An example of shutter apparatus wherein the size of an secondary aperture through which light is incident upon a photoresponsive element changes with the size of a primary aperture that admits light to a photosensitive film for exposure thereof is disclosed in Japanese Patent Publication No. 57-10406.
In this shutter apparatus, the exposure amount through the primary aperture is controlled referring to the amount of light received by the photoresponsive element through the secondary aperture. A drive device comprising an electromagnetic actuator and a return spring is adapted to drive a pair of shutter blades for reciprocative movement of these blades. Each of the shutter blades is formed with a pair of openings one of which defines a primary aperture and the other defines a secondary aperture. With the reciprocative movement of the shutter blades, overlapping portions of the openings for the primary aperture forms an exposure opening, and overlapping portions of the openings for the secondary aperture form a detection opening.
During an opening process of the shutter blades, the amount of light received through the secondary aperture is integrated, and if the obtained integral value reaches a predetermined threshold value, the electromagnetic actuator reverses the moving direction of the shutter blades, so as to provide a short closing process in which the exposure aperture is closed or eliminated at the maximum speed.
Examples of camera apparatus for taking pictures wherein an electronic flash (strobe) is automatically fired in association with the operation of the above type of shutter apparatus are disclosed in Japanese Patent Application Laid-open Nos. 52-124332 and 52-139419 and Japanese Patent Publication Nos. 60-60046 and 3-608. In these examples, when pictures are taken at night with no ambient light or an insufficient level of the ambient light, or when pictures are taken with back light where the ambient light is at a sufficient level but a main subject is relatively dark compared to its background, the total exposure amount of the film is automatically controlled by adding an amount of exposure of the film to the reflected strobe light to an amount of exposure of the film to the ambient light.
In the camera apparatus disclosed in Japanese Patent Application Laid-open Nos. 52-124332 and 52-139419, the strobe is fired at the same time that the shutter is reversed when the integral value of the output of the photoresponsive element reaches the first threshold value in the opening stroke of the shutter apparatus. After the firing of the strobe, if the integral value reaches the second threshold value that is higher than the first threshold value, the strobe light is quenched.
In another camera apparatus disclosed in the above-identified publications, when the integral value reaches the first threshold value, the strobe is fired for a period of time that is proportional to the period from the start of the exposure to the time when the integral value reaches the first threshold value. If the ambient light is at a low level and the integral value does not reach the first threshold value, the shutter is forced to be reversed at a predetermined limit time, and the strobe is fired at the same time to keep emitting strobe light for a predetermined period of time.
In the camera apparatus shown in Japanese Patent Publication No. 60-60046, the strobe is fired the moment that the exposure opening having a size corresponding to the camera-to-subject distance is formed in the opening stroke of the shutter. Then, the output of the photoresponsive element except that generated during the strobe firing period is integrated, and, when the integral value reaches the threshold value, the shutter is reversed and the exposure opening is closed.
In the camera apparatus shown in Japanese Patent Publication No. 3-608, the strobe is fired if the integral value of the output of the photoresponsive element reaches the first threshold value, and the shutter is reversed if the integral value reaches the second threshold value higher than the first threshold value after the strobe is fired. Then, the strobe light is quenched if the integral value reaches the third threshold value higher than the second threshold value.
In the conventional shutter apparatus, after the electromagnetic actuator starts driving the shutter in the closing direction while the shutter is being biased by the return spring and moved in the opening direction, a little delay time arises until the inertia of the shutter in the opening direction is canceled so that the shutter is completely shifted to the closing stroke. If this delay time varies, the exposure amount of the photosensitive film all over the exposure time varies to a large extent. This is because the exposure opening has its maximum size during exposure at around the time when the shutter is reversed, and a large difference in the exposure amount occurs even with a little difference in the delay time.
Also, the shutter is opened at a constant speed irrespective of the level of the ambient light. If the ambient light is at a high level, therefore, the exposure amount of the film is rapidly increased after the start of the exposure, resulting in a considerably reduced exposure time and a large difference in the exposure amount even with a little difference in the exposure time.
Since the shutter blades are biased or driven with the maximum output of the return spring or electromagnetic actuator, a variation in the frictional force in the shutter mechanism in each exposure operation directly leads to an error in the speed of the shutter blades, resulting in an error in the exposure amount in each exposure operation.
In the shutter apparatus disclosed in Japanese Patent Publication No. 3-5726, on the other hand, the shutter is driven by a step motor, such that the area or size of the exposure opening may be changed according to a desired function, from the start to the end of an exposure operation. In this apparatus, the rate of change in the area of the exposure opening is controlled based on the level of the ambient light measured before the start of the exposure operation, so that the exposure time is controlled to a substantially constant value even if the level of the ambient light varies. Where the ambient light is at a high level, therefore, the exposure opening is slowly opened to the smaller maximum aperture value, and then slowly closed, thus reducing an error in the exposure amount that occurs upon reversal of the shutter.
This shutter apparatus, however, requires an expensive shutter drive mechanism including the step motor and an expensive motor control circuit, and also requires an expensive control circuit which includes a memory for storing the same number of drive programs as that of combinations of the level of the ambient light and other parameters, and a circuit for selecting one of the drive programs. Further, since the motor is exclusively driven irrespective of the actual position of the shutter, errors may arise due to the frictional force of the mechanism, and a high-grade support mechanism or lubricating mechanism will be needed to avoid such errors. These expensive components increase the cost of the shutter apparatus, making the resulting product (camera) only available at a high cost.